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Author Topic: Can a radio signal be blocked by a mountain?  (Read 15179 times)

Offline labview1958

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Can a radio signal be blocked by a mountain?
« on: 14/05/2009 10:03:38 »
Can a radio signal be blocked by a mountain. Can radio waves cast a shadow when it hits like a light wave. Is there a signal in the shadow region.

Mod edit - formatted the subject as a question.  Please do this to help keep the forum tidy.
« Last Edit: 14/05/2009 12:42:49 by BenV »


 

lyner

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Re: Can a radio signal be blocked by a mountain?
« Reply #1 on: 14/05/2009 12:20:08 »
Waves are always diffracted when they pass an obstruction. If the obstruction (mountain, person or screwdriver) is much bigger than the wavelength involved then the diffraction pattern will be very sharp - you get a hard shadow when you stand in front of a point source of light and so will a screwdriver. UHF Television signals, being the same sort of wavelength as your dimensions, will spread out after passing you; after they've gone past by a metre or so, it is as if you hadn't been there. Those signals, when they come to a mountain, will be blocked very effectively - beacuse it's so big. But a long wave radio signal (1500m wavelength) will diffract round a mountain.
Microwaves will be blocked by your body but may find their way around a screwdriver.
Diffraction always happens when any waves pass through gaps and around obstacles; it's just a matter of degree.
see http://en.wikipedia.org/wiki/Diffraction
 

Offline labview1958

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Can a radio signal be blocked by a mountain?
« Reply #2 on: 14/05/2009 17:31:20 »
"If the obstruction (mountain, person or screwdriver) is much bigger than the wavelength involved then the diffraction pattern will be very sharp - you get a hard shadow when you stand in front of a point source of light and so will a screwdriver."

In a ripple tank experiment small obstruction give a more pronounced diffraction pattern than a large obstruction. It seems that the experiment contradicts the above statement.
 

Offline syhprum

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Can a radio signal be blocked by a mountain?
« Reply #3 on: 14/05/2009 22:41:40 »
Marconi had difficulty obtaining backing for his foolhardy scheme to transmit low frequency radio signals from Poldhu to Newfoundland but he played a hunch and built a high power transmitter and by a combination of diffraction and ionospheric reflection the signals got thru and ushered in the modern era of telecommunications.
 

lyner

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Can a radio signal be blocked by a mountain?
« Reply #4 on: 15/05/2009 00:26:30 »
"If the obstruction (mountain, person or screwdriver) is much bigger than the wavelength involved then the diffraction pattern will be very sharp - you get a hard shadow when you stand in front of a point source of light and so will a screwdriver."

In a ripple tank experiment small obstruction give a more pronounced diffraction pattern than a large obstruction. It seems that the experiment contradicts the above statement.
No, it's precisely in agreement. Long wavelength / small object gives a fuzzy edge and vice versa. 'Sharp' means the light doesn't 'bend' a lot.
 

Offline labview1958

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Can a radio signal be blocked by a mountain?
« Reply #5 on: 15/05/2009 09:16:43 »
If a transmiter is placed on one side of the mountain , is there a  blind spot on the other side.
 

lyner

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Can a radio signal be blocked by a mountain?
« Reply #6 on: 17/05/2009 00:28:01 »
Yes - unless you have a Long Wave transmitter and there isn't a cheap way to make a good Long Wave antenna to transmit with.
Actually, on the whole, it is good that radio waves are usually blocked by terrain. It makes it much easier to plan a broadcast network because it limits the service areas of transmitters - avoiding interference between them.
 

Offline labview1958

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Can a radio signal be blocked by a mountain?
« Reply #7 on: 17/05/2009 07:33:42 »
Is it possible that a ling wave cause space to bend around a small obstacle. Thus the Long wave appears to bend around a small obstacle. Thus a long wave has a tremendous energy that causes space to bend.
 

Offline Madidus_Scientia

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Can a radio signal be blocked by a mountain?
« Reply #8 on: 17/05/2009 08:24:34 »
The only tremendous energy you get out is tremendous energy that you put in, and it takes energy as dense as matter to bend space which is nothing compared to the energy usage of an antannae, so no I don't think so.
 

lyner

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Can a radio signal be blocked by a mountain?
« Reply #9 on: 17/05/2009 12:10:43 »
Is it possible that a ling wave cause space to bend around a small obstacle. Thus the Long wave appears to bend around a small obstacle. Thus a long wave has a tremendous energy that causes space to bend.

Long wave equals low frequency equals LOW photon energy, surely?
 

Offline syhprum

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Can a radio signal be blocked by a mountain?
« Reply #10 on: 17/05/2009 12:27:46 »
I think the cause of confusion is the high power that longwave radio stations find it necessary to feed into their antennas, I believe Moscow runs about 500KW while the Americans working at 16KHz for submarine communication where antenna efficiency is very low put out several MW. 
 

Offline labview1958

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Can a radio signal be blocked by a mountain?
« Reply #11 on: 18/05/2009 09:31:47 »
Can diffraction be explained as the incoming wave bending space around an obstacle. Thus  as to appear that it bends around an obstacle.
 

lyner

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Can a radio signal be blocked by a mountain?
« Reply #12 on: 18/05/2009 12:44:29 »
I think the cause of confusion is the high power that longwave radio stations find it necessary to feed into their antennas, I believe Moscow runs about 500KW while the Americans working at 16KHz for submarine communication where antenna efficiency is very low put out several MW. 
And the reason they need high transmitted power is that they are used for long range communications. The inverse square law applies -so high power is needed. Also, an ideal transmitting antenna would be, for example, a quarter wavelength monopole over a large perfectly conducting plate. This would involve a mast 400m high. Too expensive, so they use a shorter structure, sometimes a T shaped radiator (with wires stretched between two much shorter masts at the BBC Droitwich R4 transmitter) and a cheaper system of wires in the ground, This can be 'tuned' or matched to get the best performance but the inherent resistance of the system (ground conductivity etc) means that a lot of the transmitter power is just wasted.

Submarines trail immensely long wire antennae (kms, I believe) behind them  but, as they are in a conducting medium (seawater), a lot of the power is absorbed before it gets into the 'aether'. There is also the problem that it is horizontally polarised.
 

lyner

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Can a radio signal be blocked by a mountain?
« Reply #13 on: 18/05/2009 12:45:49 »
Can diffraction be explained as the incoming wave bending space around an obstacle. Thus  as to appear that it bends around an obstacle.
If it "bent space" as you imply, then wouldn't we all see the effect and wouldn't aeroplanes have to fly a different course? What's wrong with the current explanation?
 

Offline labview1958

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Can a radio signal be blocked by a mountain?
« Reply #14 on: 19/05/2009 09:49:01 »
I believe that space can be bent by wave obstacle interaction. The presence of an obstacle has an effect on the wave direction. How exactly to explain the diffraction pattern observed.
 

lyner

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Can a radio signal be blocked by a mountain?
« Reply #15 on: 19/05/2009 12:03:33 »
The space in a given region is 'experienced' independently by many observers at once - for instance, Electromagnetic waves of all frequencies can pass through the same region of space at once without affecting each other (this is called superposition and applies for any linear medium). So if it were as you say, they would affect each other; superposition would not happen. How can you insist that things are that way round?
Where does you approach take us?
 

Offline Raghavendra

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Can a radio signal be blocked by a mountain?
« Reply #16 on: 20/05/2009 06:50:28 »
mountain doesn't block radio signals,but there will be disturbance in signal, and the energy is lost.....
 

lyner

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Can a radio signal be blocked by a mountain?
« Reply #17 on: 20/05/2009 22:45:45 »
mountain doesn't block radio signals,

Tell that to people in Wales who can't get TV where they live!
 

Offline syhprum

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Can a radio signal be blocked by a mountain?
« Reply #18 on: 20/05/2009 23:01:42 »
The high layers of the atmosphere do not always act as a linear medium powerful transmissions can cause heating of the ionosphere and produce cross modulation
 

lyner

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Can a radio signal be blocked by a mountain?
« Reply #19 on: 21/05/2009 14:30:59 »
True, but the mountain is not actually failing to block such signals. The ones that 'get through' are just not going in the direction of the mountain. Using a mirror to see round someone would not be used as an argument to say that were not blocking out the light. (Which is why I didn't introduce ionospheric propagation into the topic.)

 It's not the heating that causes the crossmodulation - it's a 'coherent' effect, involving fields and vectors. Heating may be a secondary result of the effect.

This thread is starting to fragment, I fear.
The actual answer to the initial section of the thread is that diffraction ideas give the right answer and the theory is pretty consistent with most other ideas. If it ain't broke, don't try to mend it - especially if you are new to the subject. "Bending Space" is a red herring.
« Last Edit: 21/05/2009 14:33:25 by sophiecentaur »
 

Online Bored chemist

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Can a radio signal be blocked by a mountain?
« Reply #20 on: 21/05/2009 19:23:58 »
Inside a mountain there's no radio reception. This is because the mountain has blocked the radio signal.
 

lyner

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Can a radio signal be blocked by a mountain?
« Reply #21 on: 21/05/2009 22:17:55 »
There is still a finite skin depth.
 

Offline techmind

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Can a radio signal be blocked by a mountain?
« Reply #22 on: 21/05/2009 23:30:41 »
If a transmiter is placed on one side of the mountain , is there a  blind spot on the other side.

In general, yes.

UHF (television, 500-800MHz, ~50cm wavelength) and mobile phone 900MHz/1800MHz/2100MHz 30cm/15cm/12cm are really blocked and are very much line-of-sight with respect to mountains.

With a powerful broadcast VHF transmitter you may get a weak signal spill into the 'shadow' of the mountain to some extent, especially if you're some way from the shadowing mountain.

Mediumwave and longwave radio signals (wavelengths of 250m to 1.5km) tend to get around British mountains reasonably successfully.


If you and your friend stood on opposite sides of a mountain with VHF/UHF walkie-talkies even of quite substantial power, you would be very unlikely to have a workable communications channel.
 

Offline labview1958

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Can a radio signal be blocked by a mountain?
« Reply #23 on: 23/05/2009 15:42:19 »
"Mediumwave and longwave radio signals (wavelengths of 250m to 1.5km) tend to get around British mountains reasonably successfully"

Energy of wave = hc/λ thus a long wave has a less energy. This less energy causes the space around the mountain to bend? The wave appears to bend (diffract) around the mountain. A short wave with more energy is not affected by the mountain thus no bending of space occurs.

To test my assumption lets sent a short and long wave together towards a mountain. The long wave will bend (diffract). The short wave will follow in the foot steps of the long wave. It also should diffract. Possible?
 

Offline techmind

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Can a radio signal be blocked by a mountain?
« Reply #24 on: 25/05/2009 11:11:24 »
Energy of wave = hc/λ thus a long wave has a less energy. This less energy causes the space around the mountain to bend? The wave appears to bend (diffract) around the mountain. A short wave with more energy is not affected by the mountain thus no bending of space occurs.

To test my assumption lets sent a short and long wave together towards a mountain. The long wave will bend (diffract). The short wave will follow in the foot steps of the long wave. It also should diffract. Possible?
What you are suggesting, in effect, is that we could 'steer' the short-wavelength wave using the long-wavelength wave.

This is precisely what sophiecentaur was refuting when he said
The space in a given region is 'experienced' independently by many observers at once - for instance, Electromagnetic waves of all frequencies can pass through the same region of space at once without affecting each other (this is called superposition and applies for any linear medium). So if it were as you say, they would affect each other; superposition would not happen. How can you insist that things are that way round?
Where does you approach take us?

Where your approach would lead us would be that the radiowaves we intentionally send for an experiment might get bent all over the place by very low frequency military submarine communications which happen to be in the air at the time. They'd also get flung about by the 50Hz fields from the electricity distribution grid. And the waves you were intentionally generating would disturb higher-frequency TV and mobile-phone signals... and even visible light would fail to travel in straight lines - being deflected fairly randomly by all our radio transmissions in the air at the time.

This isn't how it is.
In free space (and for most practical purposes in our atmosphere - minor exceptions in special cases in the ionosphere excluded) the principle of superposition applies. That is, waves of different frequencies can be sent through the same space and not affect each other in any way.

Existing explanations in terms of diffraction explain the results of waves of different wavelengths getting around (or not getting around) mountains quite satisfactorily. Your proposed theory flies in the face of evidence and raises obvious absurdities when you think through the logical conclusions.

Scientifically, you don't get anywhere clinging to pet theories which don't match reality.  ;)
« Last Edit: 25/05/2009 11:19:35 by techmind »
 

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Can a radio signal be blocked by a mountain?
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